1. Field of Invention
The present invention relates to an electrical power supply method and apparatus for providing electrical power to operate particle-charging and particle collecting electrodes of an electrostatic collecting device, such as an electrostatic precipitator, by permitting the formation within the device of an electrostatic field to electrically charge the particles to be collected and to collect the particles on the collection surfaces. More particularly, the present invention relates to an electrical power supply method for converting the power provided by a three-phase, alternating current power source to direct current power in order to provide a more uniform DC voltage potential to the collection surfaces in an electrostatic collection device for improved particle collection and improved precipitator operation.
2. Description of the Related Art
Electrostatic precipitators, and other electrostatic collection devices, have taken on considerably greater importance in recent years, particularly in view of the increased emphasis upon maintaining a clean environment. That increased emphasis includes air pollution control by maintaining clean exhausts from industrial processes that involve the combustion or other form of transformation of materials, and that result in the generation of particulate matter as a consequence of carrying out the process. The techniques and structural elements incorporated in modem electrostatic precipitators, particularly the electrical control apparatus for controlling the power provided for charging the particulate matter to be collected, as well as the power provided to the collection surfaces, have been continually refined to more completely remove particulates from stack gases, such as from chemical process exhaust gases and from fossil fuel electric generating plant exhaust gases, and also to provide longer useful operating life for the precipitator components.
The most common electrostatic collection device is an electrostatic precipitator comprising charged electrodes and grounded plates or tubes. Other electrostatic devices include electrostatic filter beds, or "rock scrubbers," wherein the collecting surfaces are the surfaces of rocks or pebbles. This invention useful in any application in which particles are removed from a gas stream by the use of electrostatic forces.
The theory behind the operation of an electrostatic precipitator involves the generation of a strong electrical field through which stack gases pass, so that the particles carried by the stack gases can be electrically charged. By charging the particles electrically they can be separated from the gas stream and collected, and thereby not enter and pollute the atmosphere. The generation of such electrical fields requires electrical power supplies that can provide a high DC voltage to charge the particulate matter and thereby permit its collection. The existing systems are based upon AC corona theory, using a single-phase transformer-rectifier set to rectify AC power to DC power and provide a high DC potential between a charging electrode, to charge the particles, and a collection surface, usually a plate, so that the stack gases are subjected to the maximum current obtainable through the gas without complete breakdown. That approach is believed to produce the maximum ionization of the particles and thereby the maximum efficiency in effecting removal of such particles.
Heretofore, the emphasis in particulate removal was placed on increasing the current flow between a grid and a plate defining the electrostatic precipitator collection surfaces, to a current level that produced a maximum of sparking between the grid and the plate. In fact, some precipitators incorporated a grid structure that contains barbed wire or special pointed rods, specifically to enhance such sparking. The sparking inside a precipitator is believed to be necessary as an indicator that the maximum possible current is being drawn, and therefore the maximum possible ionization of the gases and particles is taking place. The practice of encouraging sparking is emphasized, even though it is known that sparking produces stresses upon the electrical components of the system, and it also causes increased precipitator maintenance because of the production of agglomerated particles, sometimes called "ash balls" or "klinkers," and also causes difficulty in ensuring that the "rappers," which are devices that vibrate precipitator plates to remove collected particles, are, in fact, operative and are removing collected particulate material.
Part of the problem that results from operating a precipitator or other collection device at a level at which sparking occurs is that the automatic controller for the transformer-rectifier set must sense an arc and immediately reduce the voltage on the precipitator plate, because any spark can quickly create an arc between the plate and the electrode, with a resultant high current flow. The high current flow can cause severe damage to the precipitator grid, plate, or other collecting surface; it can cause the transformer-rectifier set to fail; it can cause the controller to fail; or it can open the overcurrent protectors that a provided in the incoming power line. Any of those incidents will cause a section of the precipitator to be temporarily off-line, with the resultant passing of the particulates into the atmosphere until the failures have been repaired. Repair can be a matter of minutes, or it can be weeks if the transformer-rectifier set or controller has to be replaced.
Transformer-rectifier sets in most prevalent use are operated based upon single-phase alternating current as the power source. However, a single phase source does not produce the most steady high voltage field for efficiently collecting particulates. Additionally, the voltage control apparatus for such a transformer-rectifier set must be quite sophisticated to prevent component failures, it requires a high level of maintenance on the grid works, and, because it introduces inefficiencies, the control apparatus involves a higher consumption of electrical power than would otherwise be necessary. The high power consumption requires larger, more expensive transformer-rectifier sets, which thereby increases the capital cost to the owner or operator of the process equipment.
For the foregoing reasons, there is a need for an improved high-voltage power supply for electrostatic particle collection devices that overcomes the shortcomings of previous devices and provides a substantially uniform output voltage, with a minimum of voltage fluctuation or ripple.